Synchronisation between Video and UI/Image editor
I recommend you to convert the video to an image secuence, you can do it with Blender if you want, render the video without changes to a series of images. Anyway I prefer using ffmpeg to do this job, is free and you have lots of examples on Internet. 

I think the problem could be that, as the video is encoded, a frame in the middle could not exist (completely) and this is why it can't show the image.